Field of the Invention
Delta-sigma AD converters have been used for digitizing audio signals since approximately the mid-1980s. Numerous investigations and many scientific works over the last three decades have led to a growing field of use for these converters. Such a converter has a control loop whose forward path contains a loop filter and a quantizer and whose feedback path contains a D/A converter. Relatively high-order modulator structures and multibit D/A converters are now also increasingly used to try such converters for the higher-frequency range above the audio range.
Sigma-delta A/D converters sample the analog input signal at a much higher rate than so-called Nyquist converters. In this case, the amplitude resolution is traded for time resolution. A so-called decimation filter can be used, as previously, to generate a data stream with high resolution and at a low rate. The use of multibit D/A converters instead of single-bit D/A converters makes it possible to reduce the oversampling rate substantially on account of the lower quantization noise. In addition, loop filters with a relatively small phase margin and hence a relatively high low-pass filter range gain may be used, which permit an even better signal-to-noise ratio. However, the linearity is in this case essentially dependent only on the accuracy of the D/A converter, because the control loop of the modulator does not suppress the interference from the D/A converter. The oversampling during the delta-sigma modulation means that the interfering influence of the tolerance of the elements in the D/A converter on the useful signal can be significantly reduced using noise shaping. Without noise shaping, a correspondingly large number of D/A elements are selected in the D/A converter on the basis of an input signal, and their output signals are summed to form an output signal for the D/A converter.
In a manner which is known per se, the effects of production-related differences among the elements of the digital/analog converter can be significantly reduced by an appropriate element selection circuit. A corresponding method is described in Electronic Letters, Vol. 31, No. 20, Sep. 28, 1995, for example. To produce noise shaping, the D/A converter used in this context is a circuit including uniform delta-sigma modulators which are coupled through the use of a common vector quantizer. The vector quantizer ensures that the number of elements prescribed by the input signal are selected by the delta-sigma modulators, with specifically those modulators which have the largest output signal being selected. In this case, the vector quantizer also adopts the customary function of a quantizer for the delta-sigma modulation, because the feedback takes place via the output signal from the vector quantizer. To stabilize the entire system, the input signal taken for the delta-sigma modulation is the output component from the vector filter having the smallest value. The architecture presented in the prior art, with the corresponding stabilization through the use of the input signal, necessitates a high level of circuit complexity for the vector quantizer and additionally requires a minimum detection feature. In addition, the chosen structure for the delta-sigma modulators is very sensitive to coefficient errors and therefore requires coefficient multipliers having a large word length for complex filter structures.